1. Field of Invention
This invention relates to novel and improved electronic display systems, particularly those employing a plurality of display elements which are capable of exhibiting a television-type scanning raster. Included in this class of display elements is the familiar cathode-ray tube (CRT) as well as some forms of plasma displays.
Electronic display systems embodying the present invention may be used in a variety of applications. For example, an electronic display system may be used in a non-interactive application such as for the visual display of arrival and/or departure information in a transportation terminal. On the other hand, an electronic display system may be used in an operator interactive environment where an input device, typically a keyboard, is used by the operator to perform a host of operations such as editing, composing or retrieving textual material and the like. Specific applications include traditional key entry routines (the familiar keypunch, key-to-tape and key-to-disk operations) as well as more versatile source data capture operations where user oriented programs and/or programming languages allow the operator to record data records of the effects of a business transaction in real time and manipulate or process such data records.
2. Prior Art
In general, a television-type scanning raster is produced in a CRT by scanning the electron beam horizontally across the display surface to form a number of rather closely spaced and substantially horizontal scan lines. In this scanning process the beam is normally blanked and is selectively unblanked to achieve visual display of information. For the display of textual material, the character lines are generally defined by groups of scan lines with each group being defined by a sequence of scan line signals. Selective unblanking of the beam is achieved in synchronism with a number of sequences of dot signals which are generated during each scan line, the dot signal sequences defining the character spaces as well as their horizontal position on the display surface. The characters to be displayed may, for example, comprise alphanumeric symbols or other marks. Portions of each scan line in a character row or line group in the raster are selectively blanked and unblanked so that the scanning beam produces a slice of each character in a character line as the scanning beam forms a scan line across the CRT surface. The character slices in a character line group of scan lines then integrally form a complete row of characters.
Each character is represented in the electronic display system by a group of binary signals usually referred to as a character code. To generate the correct video signals at the proper time, selected ones of the character codes are shifted in sequence to a prescribed position during each scan line. In the prescribed position, the character codes are combined with the appropriate television scan line signals and dot signals thereby producing unblank signals at the proper time to provide slices of the corresponding character. Sequencing of the character codes causes the correct video signal to be applied to the CRT at the proper times in the scan lines.
Because the CRT display surface retains its illumination only for short periods of time, on the order of 20 milliseconds, the process of sequencing character codes and combining them with the dot signals must be repeated on the order of 40 to 70 times a second so as to present a nonflickering display. This repetitive process is sometimes called the refresh process.
Some prior art multiple CRT electronic display systems have used a separate refresh memory for each CRT to store a frame of character codes in the order that the corresponding characters are to be displayed on the CRT. The codes in each separate memory are read out in sequence at the appropriate times during the corresponding scan lines. The disadvantage of this type of system is the necessity for separate addressing and read-write controls for each memory.
In other electronic display systems, typified by U.S. Pat. No. 3,848,232, a single memory and a line buffer register are employed in conjunction with a single CRT. In this system strings of character codes are stored in the memory together with a display program. The commands or instructions of the program designate the order as well as the various programming attributes in which the strings of characters are to be displayed in the CRT. Prior to the scan of a character line on the CRT screen the line buffer is filled with one or more strings of characters under control of the display program. The program contains not only the vertical position (i.e., character lines) but also the horizontal position within a character line of a string of characters and various other programming attribute information, all of which complicate the program and result in the use of extra memory cycles.